The present invention relates to a process for the production of calcium formate in aqueous phase by reaction of formaldehyde with calcium hydroxide in the presence of hydrogen peroxide or by reaction of formaldehyde with calcium peroxide.
Calcium formate, the calcium salt of formic acid, is presently obtained almost exclusively as a by-product in the manufacture of the polyhydric alcohols (polyols) pentaerythritol, trimethylol ethane, trimethylol propane and neopentyl glycol. It is formed during the reaction of 3-hydroxyaldehydes obtained by aldol condensation of formaldehyde in the presence of calcium hydroxide. Accordingly, calcium formate is the oxidation product of a mixed Cannizzaro reaction. Sodium formate is also obtained as a by-product in the manufacture of polyols.
Calcium formate is used for various purposes, for example as a tanning auxiliary, for the production of formic acid, as a setting accelerator in the cement industry, as a silaging auxiliary and, to an increasing extent, as a nutritive supplement in animal feeds. The last of these applications, where calcium formate is used to improve feed utilization, to reduce digestive problems, to avoid microbial feed decay (see brochure of Degussa AG, "Calciumformiat"-Ch 608-1-105-988 DD) imposes particular quality requirements to ensure that the feed containing calcium formate is not refused by the animals.
The disadvantages of producing calcium formate as a by-product of polyol synthesis is that an increase in the production of formate can only be obtained in connection with an increase in the production of the particular polyol (which presupposes corresponding marketing possibilities for the polyol).
It is also known that calcium formate can be produced by carbonylation of calcium hydroxide (cf., for example, Gmelin's Handbuch, Vol. Ca(B), pages 161-162 and Ullmann's Encyclopedia, 4th Ed., Vol. A 12, pages 23-24). These processes generally require technically complicated reactors, high pressures, high temperatures and generally long reaction times.
The Cannizzaro reaction of formaldehyde with strong inorganic bases takes place in accordance with the following scheme: EQU Me(OH).sub.n +2n CH.sub.2 O.fwdarw.Me(OOCH).sub.n +nCH.sub.3 OH
n=1 for Me=K, Na PA1 n=2 for Me=Ca, Ba
The course of this reaction is determined by the metal hydroxide used. Numerous studies have shown that certain metal hydroxides do not promote the Cannizzaro reaction, but rather self-condensation of the formaldehyde. Although the formaldehyde molecule does not have an .alpha.-H atom and, accordingly, cannot enter into a normal aldol condensation, hydroxyaldehydes and ketones (particularly pentoses and hexoses, the so-called formoses) are formed by a substantially analogous reaction (T. Mizuno, A. Weiss, Adv. Carbohyd. Chem. Biochem. 29 173 (1974)). Formose-forming hydroxides include in particular Pb(OH).sub.2, Sn(OH).sub.2, TlOH and Ca(OH).sub.2.
Accordingly, it has not hitherto been possible to isolate calcium formates in high yields by reaction of milk of lime, for example, with formaldehyde in accordance with the following equation: EQU Ca(OH).sub.2 +4CH.sub.2 O.fwdarw.Ca(OOCH).sub.2 +2CH.sub.3 OH
because, after a certain incubation period dependent on the particular reaction temperature and on any impurities present, the self-catalytic formaldehyde condensation which begins at the same time converts any formaldehyde still present almost instantaneously into formoses in a highly exothermic reaction and thus removes it from the formate-forming reaction.
Russian patent 1474157 describes a process for the production of calcium formate by Cannizzaro reaction from calcium hydroxide and formaldehyde, in which saccharification of the formaldehyde is largely prevented by addition of the metal salts MnSO.sub.4, FeSO.sub.4, Cu.sub.2 Cl.sub.2 or Ce(NO.sub.3).sub.2. Although the yields of calcium formate obtained in this process are far better than the yields obtained in the absence of inhibitors, reaching around 89% in the best Example (Example 6), disadvantages lie in the very long reaction times (5 h), which cause poor volume/time yields, and the high dilutions which result in the cost-intensive evaporation of large quantities of water. In addition, the formaldehyde remaining in the solution (in Example 6, approximately 8% of the quantity used was not reacted) has to be separated from the calcium formate. This generally requires distillation under pressure on account of the particular properties of the formaldehyde. This incomplete reaction makes the basically simple Cannizzaro reaction unusable in this form for the synthesis of calcium formate on an industrial scale because the problem posed by the excess formaldehyde is not solved and calcium formate cannot be crystallized out from such solutions sufficiently free from formaldehyde. In addition, 2 mol methanol are obtained per mol calcium formate under the reaction conditions, leading on the one hand to a high input of formaldehyde and, on the other hand, to expense in removing the methanol from the reaction mixture.